Method of digesting fibrous material



.Dec. 24, 1940. T. DUNBAR METHOD OF DIGESTING FIBROUS MATERIAL Filed Oct. 21, 1956 Patented Dec. 24, 1940 UNITED STATES METHOD OF DIGESTING FIBROUS MATERIAL Thomas L. Dunbar, Watertown, N. Y., assignor t0 Chemipulp Process Inc., Watertown, N. Y., a corporation of New York Application October 21, 1936, Serial No. 106,893

11 Claims.

This invention relates to an improved method of and apparatus for the production of pulp.

A major object of the present invention is to provide a method of digesting fibrous materials in which the thermal and chemical values are conserved to the maximum degree.

Another object is to provide a novel method of absorbing gases evolved during a pulp digestion process.

A further object is to devise an improved system of establishing and controlling the concentration of digestion liquors utilized in a pulp process.

A still further object is to provide an improved apparatus for the digestion of pulp.

Yet an additional object is to devise a novel system of accumulation for the recovery of the chemical and thermal values of digestion liquor. With these and other equally important and related objects in view the invention comprehends the concept of digesting fibrous material with a digesting liquor containing a volatile component, and absorbing such volatile component in a novel system of difierential pressure accumulation or regeneration. The invention is peculiarly useful for operation with typical bisulphite liquor but it will be appreciated that it is equally available for utilization with any digestion liquor which contains a volatile component, such for example as sulphurous acid solutions, solutions of ammonia or nitrogen derivatives and the like.

The novel features of the invention will be more readily appreciated from a consideration of a typical pulp unit in which they are embodied, as diagrammatically shown in the single figure of the accompanying drawing. 1

As shown on the drawing, the major units comprisin the system are the digester or digesters I, the pressure accumulator 2, the pressure storage tank 3 and the pressure constant level tank 4. These major elements are specially interconnected, in a manner more fully to be described, so that a number of improved functions or results are secured, important among which is the maximum recuperation of the chemical and thermal values of the digestion liquor.

The pressure constant level tank 4 may be of any suitable design and construction and of sufficient capacity to satisfy the requirements of a particular installation. While the element is shown as a sealed vertical shell, it will be understood that it may be of any other desiredconfiguration. Essentially the element 4 comprises a pressure tight container to which is fed raw acid liquor from the acid makeup system and from which such liquor, after a certain degree of preconditioning, is fed in a special manner to the acid storage system. The raw acid from the acid makeup system is fed to the tank 4 through the line 5. The tank is provided with a sight or gauge glass 6 to indicate the liquid level therein. I If desired such liquid level may be indicated by a liquid level recorder of any approved type. Similarly the pressure obtaining in the tank is indicated and/or recorded by the instrument I. The temperature of the acid in the tank likewise is indicated by the thermometer or equivalent instrument 9.

Communication is established between the constant level tank 4 and the pressure storage tank 3 through the special circuit. As shown, this includes a gas seal conduit III in which is interposed the check valve II and which connects with the suction side of pump I2. The discharge side of pump I2 connects through line -I3, controlled by valve I4, with the low pressure eductor I5. In this eductor, as will more fully appear, acid liquor is utilized to condense and absorb low pressure gases withdrawn from the digester system. The

lower section of tank 4 is also connected to the suction side of pump I2 through the line I6 in which is interposed valve II.

In operation of the system this valve IT is cracked to a predetermined setting so as to constantly bleed a stream of liquor into the lower portion of the gas seal line at the suction side of the pump. When the pump is operated liquor, in admixture with some gas, is withdrawn from the upper portion of the tank 4 and forced through the line I3 and eductor I5 and thence through the drop leg I8 and check valve I9 to the lower section of the storage tank 3. During such operation liquor from the bottom of tank 4 isconstant- 1y bled into the intake side of pump l2 and thus serves as a constant primer for the pump, preventing any gas binding. The special gas seal connector I0 is provided to maintain a pressure on tank 4 and to prevent the pump I2 from reducin the liquid level so low that gas would be pumped out of the upper portion of the tank and back from the vent lineat the upper portion of the branch.

Further connection is established between the constant level tank 4 and the storage pressure tank 3 so as to establish a supplemental circuit. Thus the lower portion of tank 4 is connected through line 20, in which is interposed the valve 2|, to the lower end of the drop leg I8. A branch line leading from drop leg I8 to the pressure storage tank 3 is also provided with the valve 22.

In the event that it becomes necessary or'desirable to drain the pressure storage tank 3 or to cut it out of the system, valve 22 may beclosedand valve 2 I- may be opened. In these circumstances this liquor is withdrawn from the storage tank 4 and is forced through pump I2, line I3, eductor I5 and drop leg I8 back into the storage tank through the line 20 During this operation, as will be appreciated, the essential function of the eductor I5 is maintained, that is to say the liquor flowing through the line l3 and back through the drop leg ill tothe tank 4 absorbs and condenses low pressure gases evolved in and withdrawn from the digester system. Thus the tank 3 may be cut out of the system without affecting the digestion stage proper. In the event that it becomes necessary or desirable to drain the pressure storage tank 3, this may readily be done through the drainage circuit 24-25262'l.

Similarly, if desired, liquor can be drained'from the pressure tank 4 through the line 28 to line 21 and from the accumulator through line 29 controlled by valve 29' and line 21. Likewise, liquor can be drained from accumulator 2 or storage tank 3 through line 21 and circuit |2|3l8 and 20 to tank 4.

It will thus be seen that in an installation of the character thus far described the tank 4 subserves a number of useful functions. In this tank in normal operation a superatmospheric pressure of the order of 2 to 6 or more lbs. gauge is carried. As' will be seen" more fully hereinafter the upper section of the tank 4 is connected through vapor lines respectively to the digester system I and the high pressure accumulator system 2 so as to absorb low pressure gases evolved in these two units. By thus absorbing gases in .the raw liquor, such liquor is preheated and preconditioned. By operating the tank 4 under pressure the absorption of these gases can be increased. By connecting the tank 4 with the larger pressure storage receptacle 3 through the special liquid level drawofi connection, a constant level may be maintained in the tank 4 and fluctuations in the volumetric capacity of the system during operation may be taken care of in the units 2 and 3. Similarly, as will be appreciated,

- by sequentially absorbing low pressure gases in different units, 4 and 3, maintained under differential pressures, a more complete absorption of such gases and a more accurate control of acid concentration and temperature is achieved.

For the purpose of enabling the element 4 to act as a heat and chemical recuperative unit, a number of overhead connections are provided. Thus the digesters I are each provided with a vent branch 30, controlled by valve 3|, which connect with the main vent header 32. As will be observed this vent header is connected with the constant level tank 4 and terminates a short distance below the level of the liquid therein. Thus gases vented from the digester during filling or during any other stage of the operation may be passed through the header 32 and into theliquor within the tank 4 to be condensed and absorbed therein, thus directly heating the coolerliquor in the storage tank and preconditioning such liquor. Similarly the high pressure accumulator 2 is providedwith a vent line 33, controlled by valve 34. Valve 34 may be a manually operated valve but preferably is an automatic pressure relief valve whichis operative to vent gases from the accumulator when a predetermined pneumatic pressure is exceeded. Gases vented through line 33 pass downwardly into the bottom of the tank 4 to be initially absorbed therein under a substantial hydrostatic head of liquor. Such gases are similarly condensed and absorbed and thus serve to preheat and precondition the raw acid liquor flowing through tank 4.

The pressure storage tank 3, in a manner similar to the accumulator 2, is provided with an overhead vent line 35 controlled by the manual or automatic valve 36. This line connects with the vent line 31 which leads to the gas fan (not the digesters.

shown). The end of vent line 31 is connected to the upper part of the liquid level storage tank 4. In this line is interposedthe valve 38. Valve 38 may, if desired. be an automatic valve operating to vent gases from the upper part of the tank 4 through the line 31 so as to maintain any desired predetermined pressure within tank 4. Gases vented through line 31 may be disposed of in the manner known to those skilled in the art, as for example by being sent back to the acid makeup system.

The digester 3 is provided withsuitable instruments to indicate and/or record the temperature, pressure and liquid level conditions obtaining in the element. Thus this unit is provided with the pressure recorder 40, the recording thermometer 4i and any suitable type of liquid level indicating or recording device 42.

As has been indicated hereinbefore, acid accumulated in the pressure storage tank 3 may be forced, preferably in a continuous stream, to the pressure accumulator 2 and in transit is utilized to absorb and condense the high pressure gases evolved in the digester system. For this purpose liquor may be withdrawn from the lower section of the tank 3 and passed through the line to the pump 5|. Pump 5| picks up the preheated and partially conditioned sulphite liquor and forces it through the line 52 to the eductor 53. As will be observed, the line 52 is provided with two branches 54 and 55, each suitably controlled by Valves 54 and 55 respectively. The line 54 preferably terminates well within the eductor and liquor discharging therethrough aspirates and condenses high pressure gases discharged from For this purpose .each digester is provided with a branch line controlled by valve 6| which connect with the high pressure header 62. The high pressure header connects with the eductor to discharge gases and/or relief fluids therein, as more particularly explained in my copending application Serial No. 28,580, filed June 26, 1935, now Patent No. 2,165,644.

As described in such prior application, the acid forced by pump 5| through line 53 may be passed either through branch 5.4 or through branch 55, or simultaneously through the two branches to more effectively contact with and absorb gases and vapors entering the eductor through header 62. The line 52 is provided also with another branch or bypass line 56 in which is interposed the valve 51. The line 56 connects with the eductor l5. Also connected with the eductor I5 is the low pressure header 58. The low pressure header 58 is connected to each digester through the branch line 64 controlled by valve 65. Such branch line 64, upon opening the valve 65, serves to with draw low pressure gases from the digester, as for example during the latter part of the cooking cycle. Such low pressure gases pass through header 58 into the eductor and are there contacted with and absorbed by liquor flowing into the eductor respectively from lines [3 and bypass line 56. It will be seen that with the connections 52-56l5 acid can be recirculated from the pressure storage tank 3 back through the drop leg l8 into the storage tank to assist in preheating the acid coming from the pressure constant level tank 4.

Liquor forced through the line 52 after contacting with hot gases and vapors in the eductor 53 absorbs such gases and vapor thus preheating and preconditioning the acid. The acid then passes downwardly through the drop leg 10 and thence into the lower portion of the accumulator.

A valve H is conveniently inserted in this connection. If desired a second eductor (not shown) may be inserted at the lower end of the drop leg and by means of a suitable branch line and pump liquor may be withdrawn from theaccumulator and forced through the eductor back into the accumulator to supplement the condensing action of the eductor 53. The eductor 53, and the second eductor, when employed, then serve to establish a reduced pressure in line 62 tofacilitate the withdrawal of gases from the digester and to intimately contact these with incoming liquor to preheat and precondition such liquor.

The accumulator 2, in a manner similar to the storage tank 3, is provided, through suitable connections, with the pressure indicating and/or recording instrument 89, the temperature indicating and/or recording instrument 8|, and the liquid level indicator 82.

In typical operations under average conditions a pressure of from 2 to 6 or more lbs. is carried in the constant level tank 4; a presure of approximately 15 lbs. is carried in the pressure storage tank 3, and a pressure of the order of from 20 to lbs. more or less is carried in the pressure accumulator 2. By proper, choice of the valves 34, 36 and 38, as has been explained, such pressures may automatically be maintained.

The liquor passing in circuit through the receptacles 4, 3 and 2 is, as has been described, preheated and preconditioned to an optimum degree and such preconditioning treatment is achieved by the thermal and chemical values generated or evolved in the system. It will readily be appreciated that when desired such preconditioning and preheating may be supplemented at different stages of the circuit by the use of direct or indirect heaters and/or by introducing sulphur dioxide or other gas from an extraneous source at any suitable point. It is also to be observed that in view of the fact that the pressure on the liquor is progressively increased the free acid content of the liquor ultimately accumulated in the vessel 2 may be raised to a high point.

The liquor which has been accumulated in the vessel 2 is utilized for the saturation and digestion of fibrous material. For this purpose liquor may be withdrawn from the bottom of the high pressure accumulator 2 through valve 90 and line 9| by the pump 92 and forced through the branch lines 93, controlled by valve 94, into the bottom of the digester. If desired, direct or indirect heat exchange mechanism may be inter posed in the line 9| so as to raise the temperature of the acid to any desired degree. The digesters are each provided with the manhole cover 95 which may be removed for the. admission of chips, in the manner customary to the 'art. The digesters are also provided with a blowofi elbow and with steam inlet lines in the lower section thereof in the conventional manner. the digesters are each provided with the side relief -lines 96. Preferably this lineis extended downwardly to the lower portion of the digester and communicates therewith through the branch 91 in which is interposed the valve 98. A second branch line 99, having a valve I00, is connected to an upper section of the digester. By opening valve 99 and/or valve I00 it will be understood that the liquid level within the digester may be controlled at any predetermined value.

The digesters are each provided with a gauge glass and preferably with temperature, pressure and liquid level indicating and/or recording in- Similarly struments so that the conditions obtaining during the digestion may be carefully checked.

In a typical operation the unit is started by removing the manhole cover 95 on one or more of the digesters and packing the digester with chips. Thereafter the manhole cover is bolted on and valve 3| is opened, all of the other valves communicating with the digester being closed. The valves 90 and 94 are then opened and pump 92 started. Heat preconditioned acid liquor is then forced from accumulator 2into the lower section of the digester. As the level of the entering acid rises air is progressively displaced and may be vented off through lines 30 and header 32. It is preferable however to provide a separate air vent in the line 30 so that during the early stages of filling air may be displaced and vented to the atmosphere. When, as the level of the liquor rises, free sulphur dioxide or other gas begins to come off, then such air vent may be closed and the low pressure gas passed through the header 32 into the liquor within the constant level tank 4 to be absorbed therein. Since the acid forced from accumulator 2 in the digster is at an. elevated temperature, of the order of from 50 to 100 C. or more, such evolved gases contain available heat units. Such heat units are, therefore, absorbed in the cooler liquor in tank 4.

In the improved operation pump 92 is operated until the digester is filled or substantially filled with liquor. At this point the valve 3| is closed so that the digester is sealed. The pump 92 is nevertheless continued in operation so as to build up a high hydrostatic pressure in the accumulator. Thus the acid may be forced from the accumulator into the digester until a pressure of from 50 to 100 lbs. more orv less is developed. In these circumstances the chips or fibrous material within the digester are contacted with the hot acid liquor maintained under relatively high hydrostatic pressure. The acid quickly penetrates into the core of the chips, uniformly preheating and saturating them. If desired, to establish more uniform temperature conditions throughout the chips, the liquor may be recirlator.

After a predetermined saturating period the conditions may be adjusted to effect digestion of the material. The length of the saturating period may vary over a relatively wide period, depending upon 'such factors as the temperature of the acid, the pressure carried in the digester, the character of the fibrous material and the like. In ordinarycircumstances asaturatingperiod of from 2 to 4 hours more or less is recommended. When the fibrous material has been thoroughly saturated with the hot acid and the temperature is substantially uniform throughout the mass of chips, valve 99 and/or valve 98 may be opened so as to lower the level of the liquid in the digester to the desired degree. It will be appreciated that i culated to and from the digester and accumuby connecting the side line 95 to a lbw point in During the .cooking operation the desired operating pressure is maintained in the digester by relieving the pressure through the branch line 60. When the valve BI is opened the high pressure gases enter header 62 and the eductor 53 where, as has been explained, they are condensed by the cooler stream of acid flowing through line 52. Due to the condensing action in the eductor 53 a desirable low pressure is established in the header 62, thus facilitating the removal of the high pressure gases. The liquid level during the digestion period, as has been explained, is maintained at the desired value by leading off excess liquor through the line 96. At the latter end of the cook the low pressure gases may be withdrawn from the digester through the line 64 and header 58 to be contacted with and absorbed by cooler liquor in the eductor I5. Such cooler liquor may be comprised in part of that forced through line 13 from the tank 4 and in part by liquor passing through bypass line 56. At or near the termination of the cook some of the low pressure gases may be vented ofi through line 30 and header 32 to the constant level tank] to be absorbed by the liquor therein.

At the termination of the cook the pulp in the digester may be treated in any approved manner, Well known to those skilled in the art. Thus near the end of the cook the pressure within the digester may be reduced from the particular operating pressure, say of approximately 70 lbs. more or less, down to approximately 30 lbs. more or less and the digester may be blown at this pressure. The pulp may then be submitted to the usual treatments of washing, screening, bleaching and the like.

It will be observed that the improved method and apparatus herein described presents many advantages. By the utilization of the plural, difierential pressure recuperators 2, 3 and 4 provision is made for insuring optimum absorption of the difierential pressure gases withdrawn from the digesting stage and/or relieved from the accumulators. Thus, high pressure gases evolved and relieved during digestion are absorbed in the final stage of pre-conditloning, namely in the accumulator 2 where the higher pressure maintained insures maximum absorption. Likewise the low pressure gases withdrawn from the digester system are absorbed in the stream of acid liquor flowing through the eductor IE to the pressure storage tank 3 and are most effectively recovered.

Pressure control is achieved in each the recuperators 2 and 3 by the simple expedient of bleeding off excess gases and vapors but the chemical and thermal values of such gases are fully recovered by the liquor in the low pressure tank 4.

The provision of the pressure constant level tank 4, in conjunction with the differential pressure storage tank 3, also insures increased flexibility of operation as well as maximum recovery of heat and chemical units. These two elements, with their interconnected circuits, permit the accumulation of desirably large volumes of liquor under conditions of maximum heat retention and recovery. Due to the special circuits described, elements like storage tank 3 may temporarily be cut out of the system While nevertheless insuring the recuperation of the thermal and chemical values of low pressure gases by recirculating acid liquor directly to and from the tank 4 while absorbing the gases in such recirculating stream.

The utilization oi a pressure storage vessel of the type of element 4, in which a constant level may be maintained not only insures maximum a typical embodiment illustrating equivalent methods of carrying out the fundamental principles of the invention.

I claim:

1. A method of digesting fibrous material which comprises, cooking cellulosic material in a digester under superatmospheric pressure and temperature with an acid digestion liquor; forcing fresh acid liquor, from an acid make-up system, directly to a closed tank, establishing a body of the liquor in said tank under superatmospheric pressure, withdrawing liquor from the tank while maintaining a constant liquid level therein, and forcing it to a second tank in which the body of acid liquor is maintained under a higher pressure, absorbing hot low pressure fluids, evolved during a digestion operation, in the liquor in transit between the first and second tanks; withdrawing a stream of liquor from the second tank and forcing it to a high pressure accumulator and absorbing in such stream hot high pressure fluids evolved during the digestion operation.

2. ,A method of preconditioning acid liquor in a pulp digestion process which comprises, establishing a volume of acid liquor under pressure in a closed vessel, feeding raw acid liquor from an acid make-up system to said vessel, withdrawing a stream of liquor from said vessel while maintaining a substantially constant volume of liquor therein and passing such stream to a pressure storage tank maintained under a higher pressure than the said vessel, absorbing low pressure fluids evolved in a digestion process in said stream to preheat and precondition the liquor; forcing a stream of acid liquor from the pressure storage tank to a high pressure acid accumulator and 'venting gases from said accumulator to and absorbing them in the body of liquor in said vessel.

3. A method of preconditioning acid liquor in a pulp digestion process which comprises, establishing a volume of acid liquor under pressure in a closed vessel, feeding raw acid liquor from an acid make-up system to said vessel, withdrawing a stream of liquor from said vessel, while maintaining a substantially constant volume of liquor therein, and passing said stream to a pressure storage tank maintained at a higher pressure than said vessel, absorbing low pressure fluids evolved in a digestion process in said stream to preheat and precondition the liquor of such stream; forcing the stream of acid liquor from the pressure storage tank to a high pressure acid accumulator, absorbing in said stream passing to the acid accumulator high pressure fluids evolved in the digestion process, and venting gases from said accumulator to and absorbing them in the liquor in said vessel.

4. A method of preconditioning acid liquor in a pulp digestion process which comprises, establishing a volume of acid liquor under pressure in a closed vessel, feeding raw acid liquor from an acid make-up system to said vessel, withdrawing a stream of liquor from said vessel while maintaining a substantially constant volume of liquor therein and passing such, withdrawn stream to a pressure storage tank maintained under higher pressure than the said vessel; forcing a stream of acid liquor from the pressure storage tank to a high pressure acid accumulator and venting gases from said accumulator .to and absorbing them in the liquor in said vessel.

5. A method of preconditioning acid liquor in a pulp digestion process which comprises, establishing a volume of acid liquor under pressure in a closed vessel, feeding raw acid liquor from the acid make-up system to said vessel, withdrawing a stream of liquor from said vessel while maintaining a substantially constant volume of liquor therein and passing such stream to a pressure storage tank in which the liquor is maintained under a higher pressure than in said vessel, absorbing low pressure fluids evolved in a digestion process in said stream to preheat and precondition the liquor of said stream, forcing the stream of acid liquor from the pressure storage tank to a high pressure accumulator, diverting a portion of said last mentioned stream and introducing it into the stream of liquor passing from the first said vessel to the pressure storage tank.

6. A method of preconditioning acid liquor in a pulp digestion process which comprises, establishing a volume of acid liquor under pressure in a closed vessel, feeding raw acid liquor from an acid make-up system to said vessel, withdrawing a stream of liquor from said vessel while maintaining a substantially constant volume of liquor therein and passing said stream to a pressure storage tank in which the liquor is maintained under a pressure higher than that in said first vessel, absorbing low pressure fluids evolved in a digestion process in said stream to preheat and precondition the liquor of said stream; forcing a stream of acid liquor from the pressure storage tank to a high pressure accumulator and absorbing high pressure gases evolved in a digestion process in said stream; diverting a portion of said last mentioned stream and introducing it into 'the stream of liquor passing from the first said vessel to said pressure storage tank.

7. A method of digesting fibrous materials which comprises, cooking cellulosic material in a digester under superatmospheric pressure and temperature with an acid digestion liquor, establishing a substantially constant volume of raw acid liquor in a closed storage tank maintained under pressure, feeding raw acid liquor from the acid make-up system to said tank, withdrawing a stream of liquor from the tank while maintaining a substantially constant level of liquor therein and passing said stream through an eductor and thence to a second pressure storage tank in which the liquor is maintained under a higher pressure than that in the first storage tank; contacting the said stream of liquor in an eductor with low pressure fluids evolved in a digestion process; withdrawing a stream of the thus preconditioned and preheated liquor from said second storage tank and passing it through an eductor to a third storage tank in which liquor is maintained under a higher pressure than that in the second storage tank, and absorbing high pressure fluids evolved in the digestion process in said last mentioned stream.

8. A method of digesting fibrous material which comprises, cooking cellulosic material in a digester under superatmospheric pressure and temperature with an acid digestion liquor, establishing a substantially constant volume of raw acid liquor in a closed storage tank maintained under pressure, feeding raw acid liquor from an acid make-up system to said tank, withdrawing a stream of liquor from said tank while maintaining a substantially constant level of liquor therein and passing said stream through an eductor to a first pressure storage tank in which the liquor is maintained under a higher pressure than in said first tank, contacting the said stream of liquor in an eductor with low pressure fluids evolved in a digestion operation, withdrawing a stream of partially preconditioned and preheated liquor from said second storage tank and passing it through the eductor to a high pressure accumulator while absorbing high pressure fluids vented from a digester in said second eductor; diverting a portion of said last mentioned stream and passing it to said first eductor.

9. A method of preconditioning acid liquor in a pulp digestion process which comprises forcing raw liquor from the acid make-up.system to a pressure storage vessel, maintaining a substantially constant volume of liquor therein under superatmospheric pressure; withdrawing a stream of liquor from the vessel and forcing it to a storage tank in which the liquor is maintained under a higher pressure than that in the second storage vessel; withdrawing a stream of liquor from the pressure storage tank and forcing it to a high pressure accumulator, and optionally diverting a portion of said last stream to recirculate it back to the storage tank in admixture with the stream passing from the said pressure storage vessel to the pressure storage tank.

10. A method of preconditioning acid liquor in a pulp digestion process which comprises forcing raw liquor from the acid make-up system to a pressure storage vessel, maintaining a substantially constant volume of liquor therein under superatmospheric pressure; withdrawing a stream of liquor from the vessel and forcing it to a storage tank in which the liquor is maintained under ahigher pressure than that in the secondstorage vessel; withdrawing a stream of liquor from the pressure storage tank and forcing it to a high pressure accumulator, and optionally diverting a portion of said last stream to recirculate it back to the storage tank, and utilizing such diverted stream, in admixture with the stream passing from the saidpressure storage vessel to the said pressure storage tank, to condense and absorb low pressure fluids vented during a digestion oper, ation.

11. A method of preconditioning acid liquor in a pulp digestion process which comprises, forcing raw liquor from an acid make-up system to a pressure storage vessel, maintaining a substantially constant volume of liquor therein under superatmospheric pressures, withdrawing a stream of liquor from the vessel and forcing it to a storage tank in which the liquor is maintained under a higher pressure than in the said vessel, optionally diverting said stream from the storage tank and recirculating it directly back to said vessel; withdrawing a stream of liquor from, the pressure storage tank and forcing it to a pressure accumulator in which the liquor is maintained under a pressure higher than that in the pressure storage tank and optionally diverting a portion of said last stream and recirculating it directly back to said storage vesel.

THOMAS L. DUNBAR. 

